I think he's fixed...

Well I know he is fixed, however:

I have a new addition to the house. She came to my home last summer and she is just coming into heat. Now, one of my male cats, who is fixed, shows a lot of interest in her. I am keeping him outside now because he will actually mount her.

He has a problem with spraying as well and I did read that this can be a fixed behavior - that he will do it ou of habit. Is his behavior with my new female similar to that? A fixed behavior? Or should I take him to be checked?

If the female cat is over four months, you can have he spayed and that way the problem is solved, since she won't go into heat and you won't have to lock your boy outside. It will also have numerous health benefits for her! And if you let your cats out, she will get pregnant. And then you'll have to deal with taking care of and rehoming the babies. All can be avoided by spaying the girl.

The mounting behavior is common among neutered males. If you know he's been fixed, he has been. This is weird/gross but neutered boys's little sacks will be totally empty, while a whole one has large testicles still. It's pretty obvious if they aren't fixed. It's just an instinct, and since he can still smell that she's in heat he is mounting her. He can't do anything, I don't understand why you are keeping him outside He didn't do anything wrong, and unless he was neutered in the past month he can't get her pregnant. Neutered boy cats who reached sexual maturity before their neuter sometimes mount other cats, even other neutered boys, because, well, that's what they do.

Fixed cats also spray, especially if you waited until after 4-6 months of age to fix him. He's just marking his territory. There are many posts on spraying and a forum search will yield many tidbits on how to deal with this problem.

If he was a whole male at one time and bred, he may remember that even if neutered. Some males will do this. My first cat (which was neutered but may have sired kittens) mounted my Russian Blue female first time she came into heat. It was a one time only thing (perhaps to show her what was in store later), but it was strange.

If he's neutered and has been for awhile, then don't worry. Just get her in to be spayed as soon as you can.